The parent application of Harvey et al. describes the advantages of using a harmonically mode-locked laser as an optical pulse source, particularly a source of pulses that can be used for soliton transmission. Solitons are optical pulses having a particular shape, width and energy that causes certain nonlinear effects of a single-mode fiber to compensate for the effects of chromatic dispersion in the fiber. As a consequence, each pulse can propagate great distances on the single-mode fiber while maintaining its shape and pulse width. Among other advantages of soliton transmission is the practicality of using erbium amplifiers, rather than repeaters, for amplifying the transmitted signal, and the fact that soliton transmission permits wavelength division multiplexing and polarization division multiplexing, which can further increase transmission line capacity.
The harmonically mode-locked laser described in the Harvey et al. application uses an optical pump for exciting an erbium fiber amplifier located in a closed-loop resonator path. An electrooptic modulator in the path is driven at an appropriate frequency to form the laser light into pulses having a repetition rate harmonically related to the frequency spacing between adjacent resonant modes of the closed-loop resonator path. The improvement of the Harvey et al. application is characterized in that the optical path includes a Fabry-Perot optical resonator having a free spectral range substantially equal to the pulse repetition rate of optical pulses in the ring laser. The Fabry-Perot resonator tends to equalize the energy, shape, and width of the pulses taken as the output. Unfortunately, we have found that even with this improvement, the harmonically mode-locked laser can manifest instabilities in its pulsed output.